Vibration isolation or absorption is oftentimes desirable for nulling or canceling vibrations associated with a rotating system. Such vibrations, when left unattenuated or unabated, may lead to structural fatigue and premature failure of system components. Furthermore, inasmuch as such vibrations may be transmitted through adjacent support structure to, for example, an aircraft avionics bay, areas occupied by passengers, or other components and cabin area remote from the source of the vibration which may also be subject to these same potentially damaging or disturbing vibrations (albeit perhaps lower in amplitude due to energy absorption by the interconnecting structure). Consequently, it is most desirable to isolate or absorb these vibrations at or near the source of the vibration in the rotating system
One application which best exemplifies the need for and advantages derived from vibration isolation/absorption devices is the main torque driving hub of a helicopter rotor system. Typically, the main rotor of a helicopter, which comprises a central torque drive hub member for driving a plurality of lift producing rotor blades, is subject to a variety of aerodynamic and gyroscopic loads. For example, as each rotor blade advances or retreats relative to the freestream airflow, it experiences a sharp rise and fall of in-plane aerodynamic drag. Furthermore, as the tip of each rotor blade advances with each revolution of the rotor system, the relative velocity of the blade tip approaches supersonic Mach numbers. As such, large variations occur in the various coefficients which define blade performance (e.g., moment, lift and drag coefficients). Moreover, gyroscopic and Coriolus forces are generated causing the blades to “lead” or “lag” depending upon cyclic control inputs to the rotor system. All of the above generate substantial in-plane and out-of-plane vibrations which, if not suppressed, isolated or otherwise abated, are transmitted to the cockpit and cabin, typically through the mounting feet of the helicopter main rotor gearbox.
Various vibration isolation systems have been devised to counteract/oppose and minimize these in-plane and out-of-plane vibrations. Mast-mounted vibration isolators suppress or isolate in-plane vibrations at a location proximal to the source of such in-plane vibrations whereas transmission, cabin or cockpit absorbers dampen or absorb out-of-plane vibrations at a location remotely disposed from the source. Inasmuch as the present invention relates to the isolation of in-plane vibrations, only devices designed to counteract/oppose such vibrations will be discussed herein.
Some mast-mounted vibration isolators have a plurality of resilient arms (i.e., springs) extending in a spaced-apart spiral pattern between a hub attachment fitting and a ring-shaped inertial mass. Several pairs of spiral springs (i.e., four upper and four lower springs) are mounted to and equiangularly arranged with respect to both the hub attachment fitting and the inertial mass so as to produce substantially symmetric spring stiffness in an in-plane direction. The spring-mass system, i.e., spiral springs in combination with the ring-shaped mass, is tuned in the non-rotating system to a frequency equal to N*rotor RPM (e.g., 4P for a four-bladed rotor) at normal operating speed, so that in the rotating system it will respond to both N+1 and N−1 frequency vibrations (i.e., 3P and 5P for a four-bladed rotor). N is the number of rotor blades.
While these spiral spring arrangements produce a relatively small width dimension (i.e., the spiraling of the springs increases the effective spring rate), the height dimension of each vibration isolator is increased to react out-of-plane loads via the upper and lower pairs of spiral springs. This increased profile dimension increases the profile area, and consequently the profile drag produced by the isolator. The spiral springs must be manufactured to precise tolerances to obtain the relatively exact spring rates necessary for efficient operation such that manufacturing costs may be increased. Furthermore, these vibration isolators are passive devices which are tuned to a predetermined in-plane frequency. That is, the vibration isolators cannot be adjusted in-flight or during operation to isolate in-plane loads which may vary in frequency depending upon the specific operating regime.
Another general configuration of isolator known as a “bifilar” are mast-mounted vibration isolators having a hub attachment fitting connected to and driven by the helicopter rotorshaft, a plurality of radial arms projecting outwardly from the fitting and a mass coupled to the end of each arm via a rolling pin arrangement. That is, a pin rolls within a cycloidally shaped bushing thereby permitting edgewise motion of each mass relative to its respective arm. The geometry of the pin arrangement in combination with the centrifugal forces acting on the mass (imposed by rotation of the bifilar) results in an edgewise anti-vibration force at a 4 per revolution frequency which is out-of-phase with the large 4 per revolution (or “4P” as it is commonly referred to as helicopter art) in-plane vibrations of the rotor hub for a 4 bladed helicopter. The frequency of 4P is the frequency as observed in a nonrotating reference system.
More specifically, pairs of opposed masses act in unison to produce forces which counteract forces active on the rotor hub. In FIG. 1, a schematic of a pair of bifilar masses, at one instant in time, are depicted to illustrate the physics of the device. Therein, the masses MI, MII are disposed at their extreme edgewise position within each of the respective cycloidal bushings BI, BII. The masses MI, MII produce maximum force vectors F/2, which produce a resultant vector F at the center, and coincident with the rotational axis, of the rotating system. The combined or resultant force vector F is equal and opposite to the maximum vibratory load vector P active on the rotor at the same instant of time. This condition, when the bifilar produces an equal and opposite force F that opposes the rotor load P, reflects ideal operation of the bifilar. Excessive bifilar damping or manufacturing imperfections will cause the bifilar output force F to differ from the disturbing force P produced by the rotor either in magnitude or phase best suited to nullify the rotor loads. This condition may cause unwanted fuselage vibration. It will also be appreciated that for the masses to produce the necessary shear forces to react the in-plane vibratory loads of the rotor system, counteracting bending moments are also produced. These force couples impose large edgewise bending loads in the radial arms, and, consequently, the geometry thereof must produce the necessary stiffness (EI) at the root end of the arms. As such, these increased stiffness requirements require the relatively large and heavy bifilar arms.
While the bifilar system has proven effective and reliable, the weight of the system, nearly 210 lbs, is detrimental to the overall lifting capacity of the helicopter. To appreciate the significance of the increased weight, it has been estimated that for each pound of additional weight, direct operating cost of the helicopter may increase by approximately $10,000.
Furthermore, the pin mount for coupling each mass to its respective radial arm routinely and regularly wear, thus requiring frequent removal and replacement of the cyclical bushings. This increases the Direct Maintenance Costs (DMC) for operating the helicopter, which contributes, to the fiscal burdens of the bifilar system and the helicopter.
Therefore, a need exists for an isolation system to reduce vibrations in a rotating system that isolates a wide spectrum of vibratory loads; especially large amplitude loads, minimizes system weight, reduces aerodynamic drag and reduces DMC.